The invention relates to spectroscopic systems for the analysis of small and very small quantities of substance, particularly in the HPLC range.
Spectroscopic methods are frequently employed for analyzing substances in the fields of chemistry and biology.
Reference FR 2643147 A1 discloses a process and an apparatus for spectral photometry of liquids. Radiation vertically traverses the liquid to be tested in the direction of flow thereof by means of cone-shaped bodies. For this purpose, the large end faces of the cone-shaped bodies are directed at the liquid to be tested.
References DE-U-9013325 and GB-A 2116707 disclose optical systems for testing liquids, wherein the essential optical elements used for guiding the light are lenses.
None of the cited references are directed at a process for analyzing small and very small quantities of substance, however; nor do they refer to specific problems in microanalytical procedures.
Reference U.S. Pat. No. 4,379,235 discloses the use of fiber-optical bundles in a scanner head for improving the spatial resolution of the scanner.
The inventive complex is directed to the analysis of very small quantities of substance. This automatically means that the sample spaces shrink to filament-shaped cylinders because of the largest possible path length. As there there can be no parallel irradiation, one has to rely on approximate solutions, preferred variants of which are described whithin the framework of this invention.
The simultaneous spectrometer developed by the applicant comprises a higher aperture than any other similar device and, as a result, achieves maximum energy efficiency and optimum spectral resolution. The high aperture entails one limitation: so-called "complete image formation" (in microscopy: Koehler's principle) is no longer possible with a lens system, as the spherical and chromatic errors limit the degree of transmission. (In microscopy, one can resort to immersion.) Hence, the solution resides in an aspherical mirror optical system.
FIG. 1 shows a prior art spectroscopy system wherein the spectrometer is a simultaneous spectrometer.
The core or main feature of the simultaneous spectrometer 1 is the use of self-scanning lines of diodes 2 which were developed by Snow in 1975 and comprise 512 single diodes over a length of 1.27 cm. The silicon diodes determine the effective spectral range of the simultaneous spectrometer 1 of about 200 to 1000 nm. The use of the lines of diodes 2 in a spectrometer as developed by the applicant is determined by the line geometry, a diode width of 25 .mu.m defining the width of the exit slit 3 of the spectrometer. In the formation of images subject to the minimum error rate, i.e. 1:1, this is also the width of the entrance slit 3. The 12.52 mm spectrum length is extremely short for a spectrum of analytical interest, e.g. the visible range of from 400 to 800 nm, while the bandwidth of 0.8 nm is satisfactory. Said unusually small linear dispersion signifies a very short focal distance of the spectroscopic instrument, which would primarily result in a small dispersion element. The spectral resolution (Rayleigh criterion) for the 0.8 nm bandwidth cannot be realized in this manner, however, so that solutions based on prisms are ruled out. For a grating arrangement, short bandwidth, low groove density and large grating area, i.e. a small spectrograph having an extremely high relative aperture, are required. This automatically leads to a light conductance capable of competing with conventional instruments. The afore-said requirements of the grating 5 are met by holographically generated concave gratings.
A lighting unit 6 adapted to the design of the simultaneous spectrometer 1 is shown in FIG. 1. In order that the spectrometer 1 may be utilized with the highest efficiency possible, the arrangement is basically the same as in the spectrometer; an aspherical (ellipsoidal) mirror 7 having the same aperture replaces the high-aperture hologram grating 5. So as to achieve "complete image formation", i.e. the strictly conjugate sequence of source diaphragm--lens diaphragm, etc., the mirror 7 has the dimension of the grating 5. The light source L and the image L' of the light source which is the entrance diaphragm into the measuring device at the same time, have to be very small. The light source L is required to have a luminance as high as possible. This requirement is met by xenon lamps of minimum capacity (30 to 40 W) and an illuminated area of 0.3 to 0.5 mm, for instance. Deuterium lamps with illuminated areas of 0.5 mm, high luminance and a power consumption of 35 W are available for the ultraviolet range.
Between the diaphragm 4 with the light source image L' and the entrance slit 3 of the spectrometer 1 there is the object space 8 in which directional illumination, with only a slight inclination of rays against the optical axis, ideally: telecentric illumination, of an object or a sample 9 arranged in a sample cell is required.
To achieve this aim, an optical system comprising lenses 10, 11 used to be mounted in front of and behind the sample 9. In this manner, no significant energy efficiency is achieved, however; neither is it possible to restore the aperture required for achieving the spectral resolution (Rayleigh criterion) at the entrance slit 3 of the spectrometer 1. This means that the extraordinary possibilities offered by the simultaneous spectrometer 1 cannot be utilized in practice.
Attempts have therefore been made to solve the afore-said problem by using fiber-optical light guides 12, 13 in the object space 8, as shown in FIG. 2. Such light guides are also referred to as fiber-optical waveguides.
The fiber-optical light guides 12, 13 may be rigid or flexible monofibers or fiber bundles. Fiber-optical light guides are capable of transmitting the aperture concerned, .alpha., of 30.degree. or more in the ultrioviolet region and of up to 90.degree. in the visible region without problem.
When the light is introduced in the fiber-optical light guide 12 at the location of the image L' of the light source, the light bundle leaves the light guide at the other end thereof having the same aperture and intensity distribution.
The fiber-optical light guide 12 is not capable of providing directional illumination of the sample 9 with an aperture smaller than the entrance aperture, however. In the optical sense, the exit aperture of the fiber-optical light guide 12 is a conjugate location with respect to the entrance area; however, even when there is a true optical image of the light source at the entrance, there is none at the exit, as each cross-section through the fiber-optical light guide is equivalent but not capable of forming an image. The exit area therefore has the optical effect of a hole. The light guide geometry, i.e. the aperture, is maintained, however. As the end of the fiber-optical light guide is not capable of forming an image, as mentioned before, it is not possible to generate a defined image on the basis of subsequent lens or mirror optical systems, either. One therefore has to put up with the fact that the problem of reversible aperture change cannot be solved by the combination of fiber-optical light guides with conventional lens or mirror systems, although various attempts have been made in this respect to no avail.